The present invention relates to a sensor and a method for detecting changes in the distance between a first and a second location on the basis of optics.
It is believed that there are various methods for measuring changes in the distance between movable objects. For example, some methods may involve sensors, such as strain gauges, which are based on electrical methods. Changes in electric capacitance, as well as in magnetic flux are utilized when working with small changes in length. When employing optical methods to determine linear variations there is no need for an electrically conductive connection between the points whose change in distance is to be measured. There are interferometers for small and average distances of about 1 xcexcm to 1 m, moirxc3xa9 systems, as well as transit-time measurements of light pulses. Interferometer systems may be very precise, but they may also be extremely sensitive mechanically. Also, their operation entails substantial outlay for adjustments. For that reason, it is believed that interferometer systems must be set up as substantially vibrationless systems, and they may not be simple to use, especially for detecting changes in the distance between moving objects. It is also believed that moirxc3xa9 systems are likewise precise, but, in a measuring range beyond a few centimeters, they may only be implemented at a considerable expense. Transit-time measurements of optical pulses and/or measurements of frequency shifts produced by the Doppler effect may only be accurate for large distances and may require costly measuring electronics.
The reference xe2x80x9cBerry""s phase analysis of polarization rotation in helicoidal fibersxe2x80x9d, by F. Wassmann and A. Ankiewicz, Applied Optics, vol. 37, no. 18, June 1998, discusses a method for calculating the rotation of the polarization of light, which propagates through a helically wound optical fiber. The rotation of the polarization can be utilized for implementing an optical fiber sensor which can be used to determine the size of a displacement.
The reference xe2x80x9cTwo-dimensional HiBi fiber-optic coil strain sensorxe2x80x9d, by Y. Libo and A. Farhad, Acta Photonica Sinica, vol. 26, no. 7, July 1997, vol. 26, no. 7, pages 618-622, XP 000884999, discusses that with the aid of a wound optical fiber, to measure mechanical strains, the influence of the mechanical strain on the polarization state of the light is utilized, which propagates through the optical fiber.
The U.S. Pat. No. 5,201,015 discusses a sensor for measuring mechanical strains with the aid of an optical fiber. The optical fiber has concentric windings. When a mechanical tensile stress is exerted on the sensor, the windings are elastically stretched, causing the peripheral path of the windings and, thus, also the optical path length of the light to increase in the optical fiber. The increase in the optical path length is utilized as a measure of the externally acting mechanical strain.
The U.S. Pat. No. 4,389,090 discusses a device for producing specific polarization states of light in an optical fiber. At least one region of the optical fiber is formed as a winding or coil. The polarization state of the light can be adjusted and changed by varying the spatial orientation of the winding or coils, as well as by twisting the optical fiber.
An exemplary embodiment of the present invention is directed to providing a sensor for detecting changes in distance which is technically simple and inexpensive to implement, does not require any special mechanical stability, and which can be used to precisely determine small positional changes. A further object of an exemplary method of the present invention is to provide a method for detecting changes in distance which is simple to implement.
Another exemplary embodiment of the present invention includes a sensor for detecting changes in the distance between a first and a second location having at least one substantially helically coiled optical fiber, which is able to be mechanically connected to at least one of the locations, and having a light transmitter and a detector for optical signals. In this context, the detecting device is able to generate an output signal, which is dependent upon the polarization state of the optical signal transmitted via the optical fiber. In addition, a reference optical fiber path is provided, which simulates the optical fiber and over which a second optical signal is transmitted, the optical signals transmitted over both paths being detected in a shared or in separate detecting devices so as to enable differences in the polarization state to be determined.
Another exemplary embodiment of the present invention includes a method for detecting distance variations between a first and a second location, where:
a) mechanically coupling at least one location to a substantially helically coiled optical fiber;
b) coupling an optical signal having a known polarization state into the optical fiber;
c) recording the optical signal transmitted over the connecting line in order to acquire information pertaining to its polarization state;
d) determining the change in distance from the information on the polarization state of the transmitted signal; and
e) comparing the polarization state of the optical signal following the transmission to that prior to the transmission and/or to a reference polarization state.
Another exemplary embodiment of the present invention involves the polarization of light changing in helically wound optical fibers in response to a change in the helical parameters. The polarization of the light at the output of a simple, helically coiled, optical fiber line is sensitive to movement, in particular to accordion-like movements of the fiber. This dependency of the polarization on the form of the three-dimensional (or non-planar) curve of the fiber can be used directly to measure the form, e.g., the length of the accordion-like movements of the fiber windings. The distance between any two locations can be determined by connecting them using a movable, helically wound, elastic optical fiber line.
In another exemplary embodiment of the present invention, the form dependency of the polarization state at the output end of an optical fiber is at least in part due to the considerable dependency of the fiber""s optical activity upon the exact form of its helical windings. In the first approximation, this effect is achromatic and does not result in any polarization mode dispersion. It is believed to be caused by one of the so-called optical Berry phases, the spin redirection phase. This Berry phase or geometric phase is a phase effect produced by the structure of the fiber""s space curve and not by a difference in the optical path length, as is the case with the normal dynamic phase of light. Nevertheless, geometric phases lead to the same interference effects of the light as do normal dynamic phases.
The size or magnitude of the spin redirection phase in a helically wound fiber corresponds to the solid angle xcexa9 that the k vector (k corresponds to the propagation constant xcex2 in the technical literature) wraps around on the sphere of the light-propagation orientations in the counter-clockwise direction when the light in the fiber is directed through a helical winding.
In another exemplary embodiment of the present invention, light is coupled with a defined polarization state into the coiled optical fiber and the transmitted optical signal is detected so that inferences can be drawn with respect to its polarization state or its individual polarization components after propagating through the optical fibers. From the change in the parameters of the optical signal prior to and following the transmission, or from a comparison to a reference from a calibration measurement or a concurrent reference measurement, inferences can be drawn with respect to the form or the change in the form of the wound optical fiber and, thus, also with respect to changes in the distance between locations connected thereto.
In another exemplary embodiment, for example, polarized light can be coupled into the fiber, and its polarization state or the strength of a specific polarization component can be measured once it has propagated through the optical fiber using a polarimeter or a detector having a series-connected or upstream analyzer. From knowledge of the polarizations or of individual polarization components prior to and subsequent to the transmission, conclusions can be drawn with respect to the change in polarization caused by the form and, thus, with respect to the deformation of the coils.
In another exemplary embodiment of the present invention, if the transmission signal is compared to a reference, then precise knowledge of the polarization state prior to the transmission may not be necessary. It may be sufficient if a defined initial basic situation is always at hand. The reference is constituted, for example, of a series of measured values which were acquired during a calibration measurement using the optical fibers and which specify the output signal at specific distances between the first and second location. Alternatively, a reference signal can also be produced during the measurement in that a reference path, which may simulate the wound optical fiber, likewise receives a defined optical signal, and the two transmission signals are compared to one another. For this, they are either analyzed separately and/or both intensities are compared to one another. The actual transmission signal can also be brought into interference with the reference transmission signal and subsequently can be detected in a shared detector.
Exemplary embodiments of the present invention can eliminate the need for specular surfaces or for a special mechanical stability of the system are virtually universally applicable. The launching the optical signal into the fiber should, in fact, be mechanically stable, but it can be set up separately from the system to be measured. In addition, without entailing substantial technical outlay, the sensor can be assembled from individual, inexpensive components.